1. Field of the Invention
This invention relates to gas turbine engine. More particularly but not exclusively this invention relates to a fuel injector for the combustor of a gas turbine engine.
2. Description of the Related Art
It is known in the field of gas turbine combustion to provide staged fuel systems where particular fuel injectors are used for specific engine conditions, for example low power for low speed or high power for high speed. One problem associated with the intermittent use of particular fuel injectors is that when not in use the injector can become prone to fuel carbonization in its fuel passages. This is caused by fuel remaining in the passages of the injector during its non use being heated by the combustion process such that this fuel becomes 'coked' over time. Such deposits adversely affect the performance of the engine since the fuel flowing through an affected injector will be inhibited.
Prior art proposals which attempt to overcome this problem include the provision of a fuel purge system which utilises a pressure difference in an injector by providing two interconnecting fuel passages through which fuel may be purged when required.
In particular a number of prior art proposals disclose a purging fuel passage which is sealed during normal use of the injector. This is usually achieved by use of a valve which is opened when purging is required. However it is a problem with this arrangement that the seal will leak to some extent and hence the purge fuel passage itself may be subjected to carbon build up and hence carbonisation.
Another proposal is described in U.S. Pat. No. 5,417,054. The injector described in this patent includes two fuel passages and two air passages. Under normal fuel delivery pressure both the fuel passages issue fuel, however when the fuel is shut off the inner of the two fuel passage outlets experiences a higher pressure than the outer one. Hence fuel within the passages is purged away from the higher pressure fuel exit within the nozzle.
In all the aforementioned prior art proposals the available driving pressure, essential for providing the required fuel purge, is derived downstream of swirl vanes. It is a feature of fuel injectors that a large proportion of the inlet to outlet air pressure drop is taken across the swirl vanes. Thus the pressure differential remaining for the purge process may not be enough to complete the purge operation.
It is an object of the present invention to provide an improved injector arrangement and/or to provide improvements generally.